Split clang-win job in appveyor.yml to avoid timeout

Author: pdimov

appveyor.yml

@@ -30,7 +30,12 @@ environment:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       TOOLSET: clang-win
       ADDRMD: 64
-      CXXSTD: 14,17,20,latest
+      CXXSTD: 14,17
+
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      TOOLSET: clang-win
+      ADDRMD: 64
+      CXXSTD: 20,latest
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       ADDPATH: C:\cygwin\bin;